As a conventional field effect transistor formed in a semiconductor device, there has been known a field effect transistor provided with pocket regions underneath sidewalls, respectively. The pocket regions are formed so as to have a conductivity type opposite to that for source and drain regions, respectively. A short channel effect of the field effect transistor can be suppressed by providing the same with the pocket regions. As a reference literature concerned with a field effect transistor provided with pocket regions, there are known, for example, the following Patent Documents 1 to 3, namely, JP-A 2003-17578, JP-A 2000-232075 and JP-A 1998-189951.
With the field effect transistor described above, the higher the impurity concentration of the pocket regions, the better an effect for checking the short channel effect is, and the deeper a depth at which the pocket regions are formed, the better the effect for checking the short channel effect is. Furthermore, in order to sufficiently check the short channel effect, it is desirable to form the pocket regions so as to extend up to directly underneath a gate electrode.
Then, in order to increase the impurity concentration of the pocket regions, it is sufficient to increase a dopant dose when forming the pocket regions. However, an increase in the dopant dose will cause a drawback that an increase in junction capacitance between the source and drain regions, respectively, and a substrate, and an increase in junction leak current will result. The reason for this is because the increase in the dopant dose when forming the pocket regions will result in an increase in the impurity concentration in parts of the substrate, in the vicinity of respective lower end faces of the source and drain regions.
In order to prevent the increase in the impurity concentration of the substrate, it is sufficient to form the pocket regions at a depth shallower than the respective lower end faces of the source and drain regions. However, the formation of the pocket regions at shallow depths will decrease the effect for checking the short channel effect.
Further, in order to form the pocket regions so as to extend up to directly underneath the gate electrode, tilted ion implantation has so far been employed, however, in the case of employing the oblique ion implantation when forming the pocket regions, this will cause problems in that there occur regions where no dopant is introduced due to shielding by other gate electrodes adjacent to the gate electrode, and the regions where no dopant is introduced undergo variation in area due to variation in height of the gate electrodes. In order to prevent these problems from affecting the characteristics of the field effect transistor, an ion implantation incident angle is limited to less than on the order of 30 to 40°. As a result, with a method of forming the pocket regions so as to extend up to directly underneath the gate electrode by use of the oblique ion implantation, it has been impossible to sufficiently check the short channel effect.